This invention concerns a telephone line feeding circuit equipped with an overvoltage protection device.
Telephone lines are connected to the switching network of their local exchange so that they can be interconnected. They are supplied with direct current by means of feeding circuits placed at the end of the switching network. A blocking capacitor is inserted in each wire of the line so as to prevent the flow of direct current in the crosspoints.
However, a major disadvantage of such an arrangement is that it does not protect the switching network in the case of an a-c overvoltage coming from the line: in fact, almost all of this a-c overvoltage is present at the terminals of the feeding circuit and therefore at the crosspoints of the switching network. The crosspoints risk being destroyed if they are of the semiconductor type, or at least their lifetime will be reduced if they are metallic.
In the case of d-c overvoltage coming from the line, the switching network will be protected by the blocking capacitors, but they would have to be designed to withstand a high voltage making them bulky and more costly.